Hypoxia is a critical feature of solid tumors and has profound effects on cancer biology. We have found that hypoxia is a cause of genetic instability due to transcriptional repression of several key DNA repair factors, including the DNA mismatch repair (MMR) factors, MLH1 and MSH2, and the homology-dependent repair (HDR) factors, RAD51 and BRCA1. In addition, we have identified hypoxia-induced microRNAs (specifically miR-210 and miR-373) that regulate the expression of other DNA repair factors. Moreover, there is accumulating evidence that both hypoxia and altered BRCA1 levels can influence cell differentiation. Our work so far has shown that hypoxia can inhibit differentiation in several cell culture models, and other studies have shown that BRCA1 can alter differentiation of mammary progenitor cells and breast epithelial cells. Since both hypoxia and BRCA1 can affect gene transcription, our working hypothesis is that both hypoxia and BRCA1 may regulate the differentiation of breast cancer cells via differential expression of microRNAs. Because hypoxia represses BRCA1 expression, this could provide another potential mechanism by which hypoxia promotes malignant progression by repressing BRCA1 and thus enhancing stem cell characteristics of breast cancer cells. In Aim 1 of this project, we will examine epigenetic pathways that may regulate the expression of the BRCA1 gene in response to hypoxia, with a specific focus on chromatin modifications and the factors that may mediate them. We will also test the hypothesis that hypoxia-mediated suppression can lead to long-term repression of the BRCA1 gene promoter, providing a potential explanation for the frequent finding that this gene is silenced in sporadic cancers. In Aim 2, we will investigate the role of BRCA1 in the regulation of microRNA expression in breast cancer cells and in human mammary progenitor cells under hypoxic or normoxic conditions. The functional consequences of selected microRNA changes will be tested with respect to differentiation of breast cancer cells or mammary progenitor cells and with respect to the maintenance of stemness and tumorigenicity of breast cancer cells.